Giant Iceberg Breaks Off Antarctic Glacier

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A massive iceberg, larger than the city of Chicago, broke off of
Antarctica's Pine Island Glacier on Monday (July 8), and is now
floating freely in the Amundsen Sea, according to a team of
German scientists.

The newborn iceberg measures about 278 square miles (720 square
kilometers), and was seen by TerraSAR-X, an earth-observing
satellite operated by the German Space Agency (DLR). Scientists
with NASA's Operation IceBridgefirst discovered a giant
crack in the Pine Island Glacier in October 2011, as they
were flying over and surveying the sprawling ice sheet.

At that time, the fissure spanned about 15 miles (24 km) in
length and 164 feet (50 meters) in width, according to
researchers at the Alfred Wegener Institute for Polar and Marine
Research in Bremerhaven, Germany. In May 2012, satellite images
revealed a second rift had formed near the northern side of the
first crack.

Humbert and her colleagues studied high resolution radar images
taken by the TerraSAR-X satellite to track the changes in the two
cracks, and to observe the processes behind glacier movements.

"Using the images we have been able to follow how the larger
crack on the Pine Island Glacier extended initially to a length
of 28 kilometers [17 miles]," Nina Wilkens, one of the team
researchers from the Alfred Wegener Institute, said in a
statement. "Shortly before the 'birth' of the iceberg, the gap
then widened bit by bit so that it measured around 540 meters
[1,770 feet] at its widest point."

As the
Pine Island Glacier retreats and flows out to sea, it
develops and drops icebergs as part of a natural and cyclical
process, Humbert said. But, the way the ice breaks, or "calves,"
is still somewhat mysterious.

"Glaciers are constantly in motion," she said. "They have their
very own flow dynamics. Their ice is exposed to permanent
tensions and the calving of icebergs is still largely
unresearched."

The Pine Island Glacier ice shelf, the part of the glacier that
extends out into the water, last produced large icebergs in 2001
and 2007.

The glacier is the longest and fastest-changing on the West
Antarctic Ice Sheet. While Humbert and her colleagues did not
draw direct connections between this week's calving event and
climate change, other scientists, including marine geologists at
the British Antarctic Survey, are investigating whether global
warming is thinning Antarctica's ice sheets and speeding up the
glacier's retreat.

Yet, the flow of the Pine Island Glacier may be driven by other
factors, Humbert said. The glacier flows to the Amundsen Sea at a
rate of about 2.5 miles (4 km) per year. She says whether the
flow speeds up or slows down is based more on changing wind
directions in the Amundsen Sea, and less by rising air
temperatures.

"The wind now brings warm sea water beneath the shelf ice,"
Humbert said. "Over time, this process means that the shelf ice
melts from below, primarily at the so-called grounding line, the
critical transition to the land ice."

Still, if the
glacier's flow speeds up, it could have serious consequences,
the researchers said. The Pine Island Glacier currently acts as a
plug, holding back part of the immense West Antarctic Ice Sheet
whose melting ice contributes to rising sea levels.